1. Field of the Invention
This invention relates to fiber reinforced ceramics (FRC), and more particularly to high toughness ceramic composites having metal fiber integrally anchored in a ceramic matrix, suitable for application to high temperature iron making parts such as hot rolling rolls, continuous casting rolls, and other parts, which are required to have high toughness, along with abrasion resistance and heat resistance.
2. Description of the Prior Art
Ceramics generally possess extremely attractive properties including markedly high resistances to abrasion, heat and corrosion as compared with metallic materials and a small specific gravity, so that they have long been widely utilized commercially. However, due to extremely high brittleness, their applications to structural materials and major parts have been practically limited in spite of the above-mentioned excellent properties. On the other hand, after the so-called energy crisis, there has been a growing demand for heat resistant materials with ultra-high abrasion resistance. This has caused an increase in attempts to improve the toughness of ceramics, mainly in the following two streams.
The first one, which is called fine ceramics, intends to purge the defects or impurities by using fine ceramic powder as raw material. The other one aims to improve the toughness by compounding a ceramic material with another material. In the former case, with respect to the bending strength, it has become possible to obtain about ten times as high strength as compared with conventional monotonic ceramics, thanks to the progress of the HIP or hot pressing technology. However, experiments have revealed large irregularities in strength, which is very dependent on the size of specimens.
Furthermore, the ceramic itself is extremely low in plastic deformability, and incapable of relieving a localized concentration of stress by plastic deformation in a manner similar to metallic materials. Due to brittleness of ceramics, utmost care has to be paid in designing to prevent fracture from accelerating from a small defect or impurity. For these reasons, the ceramics lack reliability as a structural material and have not yet reached the stage of replacing metallic materials.
On the other hand, with respect to the ceramic composites, initially, attempts were made to enhance the toughness by a method of compounding ceramic fiber such as carbon fiber and SiC whiskers. This method has considerably improved the toughness of composites for a matrix of glass or the like, but exhibits no marked effect for major high strength ceramics, such as Al.sub.2 O.sub.3, Si.sub.3 N.sub.4 and ZrO.sub.2. Simply speaking, this is considered to be attributed to brittleness of the ceramic fiber itself. Therefore, attempts have also been directed to compounding with ductile metal fibers, such as in the case of Si.sub.3 N.sub.4 matrix Ta composite and Si.sub.3 N.sub.4 matrix W fiber-reinforced composite. However, Si.sub.3 N.sub.4 /W FRC fails to produce the expected effect due to degradation of W fiber, which is deteriorated to form W.sub.3 Si.sub.2, as a result of the reaction with Si.sub.3 N.sub.4 during a sintering process.
On the other hand, there has been a report ("Special Ceramics 6" by Brennan J. J., pp 123-134 (1075)) that Si.sub.3 N.sub.4 /Ta FRC is markedly improved in charpy impact energy. Nevertheless, it is deteriorated in strength since Ta is far lower than the ceramic in elastic modulus. There is another problem that cracks propagate in the direction of metal fiber upon destruction, causing the ceramic to fall off and disintegrating the whole body of the composite material. Therefore, there are only few examples in which a ceramic composite is used as a structural material. Namely, the ceramic composites for structural materials have not yet reached a stage of completion.